Method and composition for phosphatizing steel under pressure

ABSTRACT

The heat and corrosion resistance of manganese phosphate coatings on ferrous surfaces can be greatly improved by modifying a conventional phosphatizing solution to reduce the ferrous ion content thereof, adding a given quantity thereto of the manganese salt obtained from an acid selected from the group consisting of citric, tartaric, gluconic, and saccharic acids and then carrying out the coating process in a closed chamber under a steam pressure in excess of one pound per square inch.

Wagner et a1.

METHOD AND COMPOSITION FOR PHOSPHATIZING STEEL UNDER PRESSURE lnventors:Linden H. Wagner, Davenport,

Iowa; Paul G. Chamberlain, Sarasota, Fla.

The United States of America as represented by the Secretary of theArmy, Washington, DC.

Filed: Aug. 31, 1971 Appl. No.: 176,690

Assignee:

US. Cl 148/6.l5 R Int. Cl. C231 7/10 Field of Search 148/6.15 R, 6.15 Z

References Cited UNITED STATES PATENTS 5/1919 Baines 148/6.15 R

[ Oct. 23, 1973 2,516,139 7/1950 Mazia 148/615 R 2,826,517 3/1958 Miller148/6.15 Z 3,116,178 12/1963 Upham 148/615 Z Primary Examiner-Ralph S.Kendall I AttorneyHarry M. Saragovitz et a1.

[57] ABSTRACT The heat and corrosion resistance of manganese phosphatecoatings on ferrous surfaces can be greatly improved by modifying aconventional phosphatizing solution to reduce the ferrous ion contentthereof, adding a given quantity thereto of the manganese salt obtainedfrom an acid selected from the group consisting of citric, tartaric,gluconic, and saccharic acids and then carrying out the coating processin a closed chamber under a steam pressure in excess of one pound persquare inch.

9 Claims, No Drawings METHOD AND COMPOSITION FOR PHOSPHATIZING STEELUNDER PRESSURE This invention relates to the formation of phosphatecoatings on ferrous surfaces and is more particularly directed toproducing such coatings in a manner which will provide an unusual degreeof resistance to heat and corrosion.

Ferrous surfaces can be satisfactorily protected against corrosion undermost conditions by any one of several well-known phosphatizingtreatments which form a relatively porous and insoluble coating capableof serving as a base for the application of such supplementary coatingsas paints, lubricating oils, and preservative compounds. This type ofcombined coating has been found to be particularly useful for thosemilitary weapon components wherein the exterior surfaces thereof mustdisplay a relatively low reflectivity to light and moderate resistanceto wear. While the elimination of the supplementary coating wouldobviously provide a highly desirable reduction in the time and costrequired to process these military components, current phosphatizingtechniques do not provide coatings with sufficient corrosion resistanceto withstand the exceptionally severe conditions under which the weaponsare frequently operated. For example, the ordinary manganese phosphatecoating currently utilized to protect the surfaces of ferrous gun partswill generally provide no more than 8 hours of protection against thestandard salt spray test required for military acceptance. Moreover,where the operation of the gun subjects the coated surfaces thereof totemperatures in excess of 275F, the corrosion resistance thereof underthe same salt spray test is drastically reduced, often to less than onehour of exposure.

Accordingly, it is an object of this invention to provide an improvedmethod for treating ferrous surfaces to form a metal phosphate coatingthereon with an unusual degree of resistance to heat and corrosion.

immersion type of phosphatizing treatment for ferrous surfaces which canbe readily performed under steam pressure in a closed and heatedchamber.

Another object of the present invention resides in the provision of amanganese phosphate coating for ferrous surfaces which will eliminatethe need for any supplementary coating of the type currently utilized tofurnish the relatively low reflectivity to light and the moderateresistance to wear required of many of the component parts of militaryweapons.

Still another object of this invention is to provide an improvedphosphatizing composition for treating ferrous surfaces to form arelatively complex combination of manganese and ironphosphates'whichproduce a tenacious coating characterized by a lesser porosity and afargreater resistance to heat and corrosion than similar coatings incurrent use.

it has been found that the foregoing objects can best be accomplished byproviding a phosphatizing bath in which the amount of iron therein doesnot exceed 1 percent and which also includes a controlled quantity ofthe salt formed by reacting manganese or the compounds thereof with apolyhydroxy acid selected from the alpha-hydroxy or carboxylic groupscontaining 4 to 6 carbon atoms with the particular atom adjacent to atleast one of the carboxyl groups being replaced by a hydroxyl groupacid. In addition, the phosphatizing treat- It is a further object ofthis invention to provide an .40

ment is carried out in a closed chamber under a steam pressure in excessof one (1) psig.

It is well known that an insoluble coating of manganese phosphate can beformed on a ferrous surface by any one of several commercial aqueoussolutions which contain a mixture of dihydrogen phosphates of manganeseand iron and a quantity of free phosphoric acid. These compositionsreact with the ferrous surface to produce a relatively porous,crystalline coating which must be additionally treated with apreservative compound to provide up to 24 hours of protection againstthe corrosive effects of the standard 5% salt spray tests set forth inASTM procedure Bl 17-61. Such protection, however, is not adequate forthose military components which will be subjected to the severecorrosive conditions encountered in a humid or marine environment.Moreover, where the coated components also involve temperatures inexcess of 275F, the degree of protection against corrosion isappreciably reduced, often to less than one hour.

In accordance with the present invention, however, investigation hasshown that the porosity of the coatings formed with conventionalphosphatizing treatments can be considerably reduced by carrying out theprocess in an autoclave under steam pressure. Since an increase in thesteam pressure provides a correspond ing increase in temperature, theformation of the phosphate coating proceeds at a faster rate andconsequently provides a finer and less porous structure than would bethe case under atmospheric pressure. It is well known that the greaterthe porosity of the coating, the greater the proportion of the surfacearea which will be exposed to corrosive deterioration. While a steampressure of 1 psig provides satisfactory results, a higher pressure willreduce the time required to form a given thickness of phosphate coating.This is a highly desirable relationship, not only for the sake ofeconomy but because the required chemical activity of some of theorganic salts of manganese is much slower than others as will be shownin the examples to be described hereinafter.

Further experimentation has indicated that a desirable reduction in theporosity of a manganese phosphate coating on a ferrous surface isconsiderably'enhanced whenever the initial quantity of the ferrous ironin the phosphatizing solution is held to a minimum. While mostcommerical solutions average about 3 percent by weight of iron,indications are that the corrosion resistance of the resulting phosphatecoatings increases in inverse ratio to the proportion of the ferrousphosphate incorporated therein. Consequently, while a certain proportionof ferrous phosphate is required to produce the necessary crystallinestructure of the coating, it has been found that the phosphatizingsolution should contain between 0.0l-l.0 percent of ferrous ironpreferably within the lower half of such range.

Furthermore, phosphate coating it is essential that the amount of ironetched from the ferrous surface to be coated also be held to a minimum.It has been found that such etching of the iron can be considerablyreduced by the'addition to the phosphatizing solution of a givenquantity of the manganese salt produced bythe reaction between the metalor a compound thereof and a polyhydroxy carboxylic acid. For the purposeof this invention, the acid is preferably selected from the carboxylicgroup containing 4 to 6 carbon atoms with the particular carbon duringthe formation of the manganese" atom adjacent to at least one of thecarboxyl groups being replaced by a hydroxyl group. Examples of acidsfalling within this classification are citric, tartaric, gluconic, andsaccharic with the chelation tendencies thereof decreasing in theparticular order in which the acids are named. For some reason not yetfully understood, the presence of these manganese saltS in aconventional phosphatizing solution modified by a reduction in theferrous iron content thereof actually inhibits the extent to which ironwill be etched from the ferrous surface being coated during theformation of such coating. As a result, the ferrous phosphate portion ofthe complex coating is considerably reduced over that of presentcommerical types of phosphate coatings and consequently produces a finerand less porous structure which is better able to resist the combinedeffects of heat and corrosion.

To form the phosphatizing solution or bath of the present invention,manganese dihydrogen phosphate, manganese carbonate, ferrous dihydrogenphosphate and phosphoric acid are placed in aqueous solution inquantities which will provide the following ingredients expressed inpercentage by weight:

The composition of the bath is formulated within these ranges to providea total acid content of from 15 to 75 points, a point being defined asthe number of milliliters of H10 normal sodium hydroxide required totitrate a 10 milliliter sample of the bath to a phenolphthalein endpoint. At the same time, the bath is also adjusted to yield a free acidcontent of l to 5 points thereby providing a total acid to free acidratio of between 5 to l and 15 to l. A limiting factor in theseadjustments is the necessity for maintaining a pH in excess of 2.0,preferably between 2.1 and 3.1. The percentages of both the manganeseand the iron are preferably limited to the lower portions of the givenranges. The bath is completed by adding from to grams per liter of oneof the aforementioned manganese salts in such quantity that the desiredacid ratio will be maintained without causing the total manganesecontent of the solution to exceed 5.0 percent by weight. Where optimumcorrosion resistance is desired, the bath is formulated to include thecitrate salt of manganese. However, while those baths which contain thetartrate and gluconate salts of manganese are operative at temperaturesonly slightly above that at which steam will be formed (212F) therebypermitting the pressure in the autoclave to be held as low as l psig,the selection of manganese citrate requires an operating temperature ofat least 260F. In order to provide such increase in temperature, thepressure of the steam within the autoclave must be raised to about 22psig. Thus, where economy of the coating process is a dominant factor,the tartrate and gluconate salts of manganese are preferred.

Examples 1 through III are illustrative of the limits to which thephosphatizing bath must be adjusted to provide optimum conditions forcoating a ferrous surface to provide an unusual degree of resistance tocorrosion when subjected to ASTM procedure Bl l7-6l salt spray corrosiontest, even after the coating had previously been heated to a temperatureas high as 450F.

Example I A manganese phosphatizing bath was prepared to provide anaqueous solution formulated to provide:

To each liter of this solution was added 14 grams of manganese citrate.The resulting bath was then placed in an autoclave containing the steelspecimens to be coated and then heated for a total of 43 minutes toprovide a steam pressure of 22 psig which was maintained for 15 minutes.The coated specimens were removed from the autoclave and separated intotwo groups. Those in the first group were tested for their resistance tosalt spray corrosion in accordance with the method described in ASTMprocedure Bl 17-61 for a total of 500 hours of continuous exposurewithout any appearance of rust. The specimens in the second group wererinsed, dried, and then heated in an oven for one hour at a temperatureof 450F. When subjected to the same salt spray testing as used for theunheated specimens, the heated specimens showed no indication of rustingafter 180 hours of exposure.

Example ll A manganese phosphatizing bath was prepared in accordancewith the following formulation:

Free Acid (FA) 2.9 points Total Acid (TA) 14.9 points Ratio of TotalAcid to Free Acid (TA/FA) 5.1 to 1 Iron 0.05% by weight pH 3.1

The bath was modified by adding 10 grams of manganese tartrate for eachliter of solution present and then placed in an autoclave containing thesteel specimens to be coated. During a processing period of 30 minutes,the autoclave was heated to provide a steam pressure therein of l psigwhich was maintained for l5 minutes. After removal from the autoclave,some of the specimens were tested for their resistance to the corrosiveeffects of the type of salt spray atmosphere specified in ASTM procedureB1 l7-6l. No rust was found after a total of 720 hours of continuousexposure. The remaining specimens were rinsed, dried, and then heated inan oven for one hour to a temperature of 450F. These specimens withstoodthe same duration of salt spray testing as the unheated specimenswithout evidence of rusting.

EXAMPLE Ill A manganese phosphatizing bath was prepared in accordancewith the following formulation:

Free Acid (FA) l.6 points Total Acid (TA) l3.l points Ratio of TotalAcid to Free Acid (TA/FA) 8.2 to l Iron 0.05% by weight pH 2.9

This formulation was thereafter modified by adding thereto 10 grams ofmanganese gluconate per liter of solution and placed in an autoclave.The latter was then heated for a total of 43 minutes during which 1 psigof steam pressure was maintained for a period of minutes. Thereafter,several of the specimens were subjected to a salt spray corrosionatmosphere of the type specified in ASTM procedure B1 17-61 for a totalof 720 hours of exposure without any signs of rusting. The remainingspecimens were rinsed, dried, and heated in an oven for one hour at 450Fand then subjected to the same salt spray testing as the unheatedspecimens for 380 hours of exposure without any evidence of rusting.

Although none of the foregoing examples were tested to the point ofactual rusting, it should be pointed out that manganese phosphatecoatings produced in accordance with this invention are fully capable ofwithstanding more than 1000 hours of exposure to a corrosive salt sprayatmosphere even when subjected to temperatures as high as 450F. This isa phenomenal improvement over the corresponding 1% to 2 hours ofprotection afforded by comparable coatings produced with commercialphosphatizing baths. Moreover, the coating produced in accordance withthis invention provides the non-reflective surface and moderate wearresistance required of military weapon components. Furthermore, thegreater density of the coating over comparable commercial coatingspermits a significant decrease in the thickness required to provide agiven corrosion resistance. As a result, the processing of the presentinvention may be accomplished more rapidly and therefore moreeconomically.

While the various formulations of the phosphatizing bath and theprocessing techniques are limited to certain preferred relationships, itwill also be obvious to persons skilled in the art that variousmodifications to the examples and details described herein are possiblewithout departing from the spirit and the scope of the presentinvention. Therefore, it is desired that the present invention shall notbe limited except insofar as it is made necessary by the prior art andby'the spirit of the appended claims.

We claim:

1. A method for treating ferrous surfaces to form a manganese phosphatecoating thereon characterized by the ability to withstand the corrosiveeffect of a 5% salt spray test for more than 750 hours, said methodcomprising the steps of,

formulating a manganese phosphatizing bath containing from 0.01 to 1.0percent of ferrous iron, adding from 0.1 to 2.0 percent by weight of themanganese salt of a carboxylic acid selected from the group containing 4to 6 carbon atoms with the particular carbon atom adjacent to at leastone of the carboxyl groups being replaced by a hydroxyl group, said saltbeing added without exceeding a total of 5.0% manganese in said bath,and subjecting said bath and the ferrous surfaces to be coated to asteam pressure in excess of atmospheric pressure for a period of from 1to 60 minutes selected in inverse ratio to the particular steam pressurebeing utilized.

2. A method for treating ferrous surfaces to form a manganese phosphatecoating thereon characterized by the ability to withstand, subsequent tobeing heated to a temperature as high as 450F, the corrosive effect of a5% salt spray test for more than 750 hours, said method comprising thesteps of,

formulating a manganese phosphatizing bath wherein the ratio of totalacid to free acid is between 5 to l and 15 to l and the pH ratio is inexcess of 2.0,

adding from 0.1 to 2.0 percent by weight of the salt obtained byreacting manganese with an organic acid selected from the carboxylicgroup containing 4 to 6 carbon atoms with the particular carbon atomadjacent to at least one of the carboxyl groups being replaced by ahydroxyl group, the amount of said added manganese salt being selectedto maintain said acid ratio and said pH without causing the totalmanganese content of said bath to exceed 5.0 percent,

confining said bath and the ferrous surfaces to be coated within asealed container, and

heating the interior of said container to provide a steam pressure ofbetween 1 and 22 psig for at least 15 minutes.

3. A method for treating ferrous surfaces to form a manganese phosphatecoating thereon adapted to resist the corrosive effect of a standard 5%salt spray test comprising the steps of,

formulating an aqueous manganese phosphatizing bath to include between0.01 and 1.0 percent of ferrous iron,

enriching the bath with from 0.1 to 2.0 percent by weight of themanganese salt of a carboxylic acid selected from the group consistingof citric, tartaric, gluconic, and saccharic acids,

confining the bath and the ferrous surfaces to be coated thereby withinan autoclave, and

subjecting the contents to steam pressure in excess of one psig for atleast 15 minutes during the formation of the phosphate coating on theferrous surfaces.

4. An aqueous bath for coating ferrous parts with a low porosity,non-reflective metal phosphate surface, said bath consisting essentiallyof,

a mixture of manganese and ferrous dihydrogen phosphates providing from0.1 to 3.0 percent by weight of manganese and from 0.01 to 1.0 percentby weight of ferrous iron,

sufficient phosphoric acid to provide a total acid to free acid ratio ofbetween S to 1 and 15 to 1 and a pH between 2.1 and 3.1, and

a manganese salt formed from a carboxylicacid selected from the groupconsisting of citric, tartaric, gluconic, and saccharic acids, said saltbeing added to the bath in a percentage by weight which will maintainthe existing acid ratio and pH without increasing the total manganesecontent above 5.0 percent.

5. The phosphatizing bath defined in claim 4 wherein said salt ismanganese citrate.

6. The phosphatizing bath defined in claim 4 wherein said salt ismanganese tartrate.

7. The phosphatizing bath defined in claim 4 wherein said salt ismanganese gluconate.

8. In an aqueous manganese base phosphatizing bath for coating ferrousparts with a low porosity, nonreflective metal phosphate surface,

a mixture of manganese and ferrous dihydrogen phosphates providing from0.1 to 3.0 percent by weight of manganese and from 0.01 to 1.0 percentby weight of ferrous iron,

from 10 to 14 grams per liter of manganese citrate such that the totalmanganese content of the bath does not exceed 5 .0 percent by weight,and

sufficient phosphoric acid to provide from 0.2 to 4.0 percent ofphosphate while maintaining a total acid to free acid ratio of between 5to l and 15 to l and a pH of between 2.1 and 3.1.

9. The phosphatizing bath defined in claim 8 including sufficientmanganese carbonate to adjust the total acid to free acid ratio to about7.5 to 1.

2. A method for treating ferrous surfaces to form a manganese phosphatecoating thereon characterized by the ability to withstand, subsequent tobeing heated to a temperature as high as 450*F, the corrosive effect ofa 5% salt spray test for more than 750 hours, said method comprising thesteps of, formulating a manganese phosphatizing bath wherein the ratioof total acid to free acid is between 5 to 1 and 15 to 1 and the pHratio is in excess of 2.0, adding from 0.1 to 2.0 percent by weight ofthe salt obtained by reacting manganese with an organic acid selectedfrom the carboxylic group containing 4 to 6 carbon atoms with theparticular carbon atom adjacent to at least one of the carboxyl groupsbeing replaced by a hydroxyl group, the amount of said added manganesesalt being selected to maintain said acid ratio and said pH withoutcausing the total manganese content of said bath to exceed 5.0 percent,confining said bath and the ferrous surfaces to be coated within asealed container, and heating the interior of said container to providea steam pressure of between 1 and 22 psig for at least 15 minutes.
 3. Amethod for treating ferrous surfaces to form a manganese phosphatecoating thereon adapted to resist the corrosive effect of a standard 5%salt spray test comprising the steps of, formulating an aqueousmanganese phosphatizing bath to include between 0.01 and 1.0 percent offerrous iron, enriching the bath with from 0.1 to 2.0 percent by weightof the manganese salt of a carboxylic acid selected from the groupconsisting of citric, tartaric, gluconic, and saccharic acids, confiningthe bath and the ferrous surfaces to be coated thereby within anautoclave, and subjecting the contents to steam pressure in excess ofone psig for at least 15 minutes during the formation of the phosphatecoating on the ferrous surfaces.
 4. An aqueous bath for coating ferrousparts with a low porosity, non-reflective metal phosphate surface, saidbath consisting essentially of, a mixture of manganese and ferrousdihydrogen Phosphates providing from 0.1 to 3.0 percent by weight ofmanganese and from 0.01 to 1.0 percent by weight of ferrous iron,sufficient phosphoric acid to provide a total acid to free acid ratio ofbetween 5 to 1 and 15 to 1 and a pH between 2.1 and 3.1, and a manganesesalt formed from a carboxylic acid selected from the group consisting ofcitric, tartaric, gluconic, and saccharic acids, said salt being addedto the bath in a percentage by weight which will maintain the existingacid ratio and pH without increasing the total manganese content above5.0 percent.
 5. The phosphatizing bath defined in claim 4 wherein saidsalt is manganese citrate.
 6. The phosphatizing bath defined in claim 4wherein said salt is manganese tartrate.
 7. The phosphatizing bathdefined in claim 4 wherein said salt is manganese gluconate.
 8. In anaqueous manganese base phosphatizing bath for coating ferrous parts witha low porosity, non-reflective metal phosphate surface, a mixture ofmanganese and ferrous dihydrogen phosphates providing from 0.1 to 3.0percent by weight of manganese and from 0.01 to 1.0 percent by weight offerrous iron, from 10 to 14 grams per liter of manganese citrate suchthat the total manganese content of the bath does not exceed 5.0 percentby weight, and sufficient phosphoric acid to provide from 0.2 to 4.0percent of phosphate while maintaining a total acid to free acid ratioof between 5 to 1 and 15 to 1 and a pH of between 2.1 and 3.1.
 9. Thephosphatizing bath defined in claim 8 including sufficient manganesecarbonate to adjust the total acid to free acid ratio to about 7.5 to 1.